The availability of solid-state image sensors (imagers) such as MOS or CCD devices, has renewed the interest in color encoding schemes for cameras including less than three images for sensing the three colors which define an image. The inherent geometrical stability of the solid-state imager allows schemes which would be extremely difficult to realize in practice with pick-up tubes, such as vidicons or saticons. In the past, many color-encoding filters have been developed. However, in general, these prior art filters have resolution and crosstalk problems which make them unsuitable for use in some high quality, solid-state imager camera systems.
For example, in a frame-transfer CCD imager (also known as a field-transfer CCD imager), the whole imaging area is photosensitive. The individual photosensitive collection sites, called picture elements or "pixels" are defined in the horizontal direction by vertical channel stops and in the vertical direction by horizontal gates having 2, 3 or B 4 phase signals applied thereto. Interlace of the even and odd fields, which cover separate areas in the image for a normal TV signal, is achieved by vertical overlap of pixels in alternate fields. Illustratively, FIG. 1 shows a portion of a frame-transfer imager 10 with the dotted horizontal lines showing vertical direction boundaries for even fields and the solid horizontal lines showing vertical direction boundaries for odd fields. TV line numbers are shown to the left and right of imager 10. A pseudo-interlace is obtained by applying the multiphase signals to the horizontal gates so as to define the pixel structure in the two fields with a vertical offset corresponding to one unit of vertical resolution therebetween. This mode of operation is equivalent to summing over two units of vertical resolution from adjacent lines where a pixel is the combination of two units of vertical resolution in the vertical direction in each field. The vertical resolution limit is not affected by this, but contrast is reduced for vertical spatial frequencies near the Nyquist limit of the vertical sampling.
It should be noted that the present invention is also useful with solid-state imagers other than frame-transfer CCDs, for example, with imagers which allow operation with non-overlapping sampling elements such as an interline-transfer CCD imager on an MOS diode array imager. The detailed discussion of the present invention, however, will be devoted to where the invention is particularly useful, i.e., in a frame-transfer type of imager.
The overlapping interlace mode of a frame-transfer CCD which does not allow access to single units of vertical resolution, represents a stringent boundary condition for which many prior art color-encoding patterns are not well suited. For instance, a classical example of a prior art color-encoding pattern, the so-called Bayer pattern is shown in FIG. 2a, wherein R, G and B refer to red, green and blue colors of the color transmissive filter elements, respectively. When used with a frame-transfer CCD, only two types of signals, R+G and B+G, would be alternately generated. As is well known, to solve for three unknown quantities (R, G, and B), three equations are required. Thus, for decoding a full (three) color signal, a minimum of three different color signals are required. Since the Bayer pattern only generates two types of color signals it is not suitable for use with the frame-transfer type of imager.
One class of color-encoding filter patterns well suited for frame-transfer CCDs are described in my prior U.S. patent application Ser. No. 559,460 filed Dec. 12, 1983, jointly with R. Morf and E. Heeb, entitled "Encoding Pattern for Single Chip CCD Camera Processing Scheme" and assigned to RCA Corporation. These patterns are referred to therein as "shift" patterns. Shift patterns have a first row of color filter elements which define a sequence of P color elements. Each sequential row is formed by filter elements which repeat the prior color sequence but shifted in the row direction with respect to the prior row by a certain number of filter elements S, wherein O&lt;S&lt;P. For constructing a color camera, each element of the filter is aligned with the pixel structure of the camera imager. When used in conjunction with the previously described frame-transfer CCD imager, the vertical dimension of the individual filter elements are two units of vertical resolution high.
FIG. 4a of our prior application is reproduced here as FIG. 2b and illustrates a shift pattern of the type wherein P=6 and S=2. This pattern contains three color filter elements, green (G), cyan (Cy) and white (W), having a sequence G,Cy,G,G,W,G. The sequence of each sequential row is shifted two elements to the left from the sequence of the preceeding row. Thus, the pattern repeats in the vertical direction after P/S or three rows. If P/S is not an integer, the pattern would repeat vertically after P rows.
It has been found that when shift patterns of the type illustrated in FIG. 2b are used in conjunction with a CCD imager having a relatively high vertical crosstalk (light energy illuminating one filter row is received by collection sites of the imager as if the light energy also illuminated the vertically adjacent filter rows) only two independent color signals can be decoded. For example, if the vertical crosstalk is 331/3%, when light is directed at the second filter row, it is received by the lines of collection sites of the imager as if the filter rows both above and below the second row were also illuminated. This causes 662/3% of the light which illuminated the second filter row to be received in the line of imager collection sites optically aligned with the second filter row and the remaining 331/3% of the light which illuminated the second filter row to be received in adjacent lines of image collection sites. Thus, as noted by inspection of FIG. 2b, since the filter pattern repeats in the vertical direction every third line, irrespective of which filter element is illuminated, only two color signals are provided by the filter i.e. GGW and GGCy. In practice, this type of pattern is noticeably degraded at CCD vertical crosstalk levels exceeding 25%, which crosstalk levels can not be reduced economically with today's technology, as far as known by the inventor herein. A color filter pattern described in our prior application and illustrated herein as FIG. 2c has a vertical period of four filter elements and is less sensitive to vertical crosstalk due to the greater number of different vertically adjacent filter elements. Unfortunately, as noted in our prior application, no analog color decoding scheme has been found which is suitable for processing the signals generated by this pattern. Although a generalized digital signal processing circuit such as described in our prior application can be utilized to provide satisfactory decoding, it would require a substantial number of digital integrated circuits. Consequently the decoding circuitry would require a substantial volume and consume a significant amount of power, which requirements are not compatible with small size and light weight of a portable color camera, for which the color encoding filter/CCD imager combination is ideally suited.
Therefore, it is desirable to provide a shift type color encoding filter pattern having a color sequence which has a vertical repetition sequence of not less than four rows and which lends itself to relatively simple and therefore economical analog decoding.